I tested Steam on Ubuntu and Kubuntu with my ATI 5670 card and I think this system is really horrible. My favorite Linux is Arch with KDE and the Catalyst 12.11 Beta 8 working perfectly, but on Ubuntu only the Beta 11 working with Steam (Beta 8 give Segment fault error) and I have lower FPS.

I tested both 12.04 and 12.10, but I got same result. On Arch I got 50-60 FPS in SS3 (same like on Windows), but on Ubuntu I have only 25-30 and sometimes the game freeze. On Kubuntu I have 45-55 FPS, but this system is useless.

On Archlinux I use catalyst-total package from AUR. This working with Steam and no black screen bug in SS3 or TF2.